1. Field of the Invention
The present invention relates to a vibration sensing device for mounting on a building, structure, machine or the like to detect vibrations.
2. Description of Prior Art
Vibration sensing devices, often called inertia sensing devices or accelerometers are now used extensively for many operations. For example, they are used in security equipment for buildings and the like and in safety equipment for machinery and the like. These vibration sensing devices are switches operated by movement of the switch as a whole under the influence of an accelerating force, for example, any vibration, impact or the like.
Further, such vibration sensing devices are particularly useful for mounting on domestic appliances, for example, washing machines and spin dryers so that when such machines are subjected to excessive vibrations due to over or eccentric loading of the drum, the vibration sensing device can be used as a switch to cut off the supply of electricity. Similarly, such vibration sensing devices may be used to disconnect the electrical system of a vehicle, for example, in the event of a crash or can be used as a security device in the event of unauthorised interference with the vehicle.
These vibration sensing devices, in general, are frequency sensitive and may be of the type which include at least two members which form part of an electrical circuit. Vibration of the device causes the members to move relative to each other to open or close the electric circuit. More usually, vibration sensing devices comprise an electrically conductive inertia mass supported on a pair of spaced apart electrically conductive support members which together form part of the electrical circuit. Vibration of these devices causes the inertia mass to move off the support members, thereby opening the electrical circuit. These vibration sensing devices are connected to analysing apparatus and being frequency sensitive, give a frequency response dependant on the vibration to which the device is subjected to. The analysing apparatus analyses the response, and depending on the response, for example, may activate an alarm or open a switch.
It is important that security and safety equipment of this type is sufficiently sensitive to distinguish between vibrations in a building, structure or machine caused by natural phenomenon and those caused by an attempted break-in to a building or structure or the iminent distruction of a machine. Vibrations caused by natural phenomenon are, for example, those caused by traffic, environmental conditions such as storms or birds flying against a window or fence. In general, vibrations caused by attempted break-ins are caused by sawing, cutting glass, chiseling putty or a window breaking. It has been found that vibrations caused by natural phenomenon, are in general of high amplitude low frequency, in other words high energy vibrations. While vibrations caused by an attempted break-in for example are usually of high frequency low amplitude, in other words low energy vibrations.
Unfortunately, with present security and safety equipment, in order to avoid false alarms it is essential to use relatively complex, and in turn expensive analysing apparatus to adequately analyse the frequency response from the vibration sensing device. This is mainly because of the relatively low sensitivity of some vibration sensing devices. Attempts have been made to overcome this problem by providing vibration sensing devices of relatively high sensitivity. One example of such a device is disclosed in U.S. Pat. No. 4,185,180. In this device aninertia mass is provided by an electrically conductive bar supported on tracks formed by holes in a pair of spaced apart electrically conductive plates. This device has a relatively high sensitivity and also by changing the construction of the bar its frequency response can be readily and easily altered. However, relatively complex analysing apparatus is still required to analyse the frequency response from the vibration sensing device so that vibrations caused by spurious sources may be discriminated against.
Another example of such a vibration sensing device is disclosed in British Pat. No. 1,263,076. This vibration sensing device comprises an electrically conducting sphere supported normally on a seat formed by three pins or legs, two of the pins or legs forming electrical contacts such that upon dislodgement of the sphere off the seat upon vibration of the switch, a circuit between the two contacts is momentarily broken. However, this device also needs relatively complex analysing apparatus to analyse the response generated by vibrations.
A further disadvantage of known vibration sensing devices is that they can only be placed in certain orientations relative to the surface on which they are placed. For example, the sensing device of British Pat. No. 1,263,076 can only be arranged with the pins vertical and accordingly it must either be provided with an adjustable mounting plate or some other means whereby it can be positioned accurately. This presents particular problems when a vibration sensing device has to be mounted to a narrow sash bar or a window or indeed a inclined sash bar of a skylight, for example. U.S. Pat. No. 4,185,180 partly overcomes this problem in that it can be mounted in any orientation in a particular plane. However, if during mounting it is inclined at an angle in such a way that the bar is not horizontal, the vibration sensing device will not operate correctly and indeed may not operate at all.
The term structure is used hereinafter in this Specification to include, buildings, machines and the like.